One billion years ago, volcanoes bulged from the landscape and hot lava coated the future towns of Monroe and Marshville, Waxhaw and Wingate in Union County. This region is known as the state’s “slate belt” — rocks formed by volcanoes.

Where there is a slate belt, there is often naturally occurring arsenic. That arsenic can be sloughed into groundwater, where it can find its way into drinking water wells, And people who are exposed to arsenic, particularly for a long time, are at greater risk for cancer — and possibly Alzheimer’s disease.

Many people in Union County, a bedroom community of 212,000 people near Charlotte, live in not only the Slate Belt but also in rural areas that rely on private wells for drinking.

The strengthening connection between arsenic exposure and Alzheimer’s has led Duke University researchers to explore how coal ash ponds, which also contain high levels of arsenic, could be a risk factor for the disease in those living nearby.

Dr. Julia Kravchenko, an assistant professor in the Department of Surgery at the Duke School of Medicine, is researching potential links between arsenic from coal ash and the fatal brain disorder.

“We can suggest coal ash is one of the factors,” Kravchenko said at a gathering of environmental health scholars in Durham last week. “The most dangerous coal ash ponds are in the same belt of arsenic.”

Coal ash ponds and dams, southeastcoalash.org

For example, Gaston and Rowan counties, which are on the western edge of the Slate Belt, contain three coal ash ponds, and have elevated levels of arsenic, have higher than average rates of Alzheimer’s deaths, 38.1 and 45.9, respectively.

Some counties with coal ash ponds, such as Stokes and New Hanover, which are not in the Slate Belt, do not.

Arsenic in waste from poultry farms, where low levels of of the chemical are often used in feed, could also contaminate groundwater. Montgomery and Moore counties, which are large poultry-producing areas and lie in the Slate Belt, report Alzheimer’s death rates of 42.8 and 38.9

“We need to next look at co-factors and check correlations in other states,” Kravchenko said.